New mom Beyonce, who gave birth to daughter Blue Ivy in January, knew there was no time to waste in slimming down as the May dates for her four-concert weekend in Atlantic City, NJ approached. Especially when the star would be squeezing her bod into custom-made Ralph & Russo designs, including a midriff-baring, fringed two-piece adorned with Swarovski crystals.

"Y'all have no idea how hard I worked!" the 30-year-old exclaimed to her fans during her concert at Revel Beach's Ovation Hall Saturday night. "I had to lose 60 pounds. They had me on that treadmill. I ate lettuce!"

Saturday's show proved to be quite the star-studded event: Michella Obama and her daughters Malia, 13, and Sasha, 10, were in attendance. (Back in April, Beyonce wrote a heartfelt, public letter to the First Lady in which she thanked her for being a "true, strong African American woman.")

"They sat up in a private balcony with Gayle King. Michelle was jamming out to 'Crazy in Love,'" a concertgoer tells Us Weekly. "When Beyonce told the crowd to get up and jump, [Malia and Sasha] stood and started dancing and jumping. They were shouting when Beyonce asked, 'Do girls run the world?'"

"Together, all three of them did the 'Single Ladies' dance moves," the concertgoer adds.